Image Processing Apparatus, Image Processing Method and Storage Medium to Generate Subtraction Image

1. An image processing apparatus comprising: a subtraction unit configured to generate a digital subtraction angiography moving image composed of a plurality of frames, from X-ray images of an object acquired before and after injecting a radiopaque dye into the object; an evaluation unit configured to calculate evaluation values for each of the plurality of frames of the digital subtraction angiography moving image; a selection unit configured to select, by comparing the evaluation values for each of the plurality of frames, at least one frame from the plurality of frames of the digital subtraction angiography moving image; and an output unit configured to output the selected frame as a reference image.

2. The image processing apparatus according to claim 1 , wherein the subtraction unit is configured to generate each of the plurality of frames of the digital subtraction angiography moving image by subtracting a mask image from live images, the mask image being generated from an X-ray image of the object acquired before injecting the radiopaque dye, and the live images being X-ray images constituted by a plurality of frames acquired after injection of the radiopaque dye.

3. The image processing apparatus according to claim 1 , wherein the evaluation unit is configured to calculate the evaluation values from the plurality of frames of the digital subtraction angiography moving image using the number of pixels that have pixel values falling within a predetermined range with respect to a threshold.

4. The image processing apparatus according to claim 3 , wherein the evaluation unit is configured to calculate the evaluation values from the plurality of frames of the digital subtraction angiography moving image using the number of pixels that have pixel values falling within a predetermined range with respect to a threshold and that are continuously arranged.

5. The image processing apparatus according to claim 1 , wherein the evaluation unit is configured to calculate the evaluation values from the plurality of frames of the digital subtraction angiography moving image using the number of pixels at a boundary between the radiopaque dye and a background region.

6. The image processing apparatus according to claim 1 , wherein the evaluation unit is configured to calculate the evaluation values based on an imaging portion for which the plurality of frames of the digital subtraction angiography moving image have been generated, imaging technique information, imaging conditions, and a time period that has elapsed since the injection of the radiopaque dye.

7. The image processing apparatus according to claim 6 , wherein the evaluation unit is configured to acquire a frame range for the plurality of frames of the digital subtraction angiography moving image based on the imaging conditions and to calculate the evaluation values within the frame range.

8. The image processing apparatus according to claim 1 , wherein the evaluation unit is configured to calculate the evaluation values by comparing a minimum-value projection image with the plurality of frames of the digital subtraction angiography moving image, the minimum-value projection image being constituted by pixels that each have the smallest pixel value among corresponding pixels in all the plurality of frames of the digital subtraction angiography moving image.

9. The image processing apparatus according to claim 1 , wherein the evaluation unit is configured to calculate a plurality of feature amounts from the plurality of frames of the digital subtraction angiography moving image, and to calculate the evaluation values based on addition of the calculated plurality of feature amounts.

10. The image processing apparatus according to claim 9 , wherein the evaluation unit is configured to calculate the evaluation values by adding values acquired by multiplying the plurality of feature amounts calculated from the plurality of frames of the digital subtraction angiography moving image by weight coefficients through linear combination.

11. The image processing apparatus according to claim 1 , further comprising an image display unit configured to display the reference image.

12. The image processing apparatus according to claim 11 , wherein the selection unit is configured to select, using the evaluation values, more than one of the plurality of frames as reference images, and wherein the image display unit is configured such that when a plurality of frames have been selected as reference images, the image display unit displays the reference images one by one at a predetermined frame rate.

13. The image processing apparatus according to claim 1 , further comprising a storage unit configured to store the reference image.

14. An image processing method for an image processing apparatus, said method comprising: a subtraction step of generating a digital subtraction angiography moving image composed of a plurality of frames, from X-ray images of an object acquired before and after injecting a radiopaque dye into the object; an evaluation step of calculating evaluation values for each of the plurality of frames of the digital subtraction angiography moving image; a selection step of selecting, by comparing the evaluation values for each of the plurality of frames, at least one frame from the plurality of frames of the digital subtraction angiography moving image; and an outputting step of outputting the selected frame one of the frames as a reference image.

15. A non-transitory computer readable storage medium having stored therein a program for causing a computer to execute an image processing method for an image processing apparatus, said method comprising: a subtraction step of generating a digital subtraction angiography moving image composed of a plurality of frames, from X-ray images of an object acquired before and after injecting a radiopaque dye into the object; an evaluation step of calculating evaluation values for each of the plurality of frames of the digital subtraction angiography moving image; a selection step of selecting, by comparing using the evaluation values for each of the plurality of frames, at least one frame from the plurality of frames of the digital subtraction angiography moving image; and an outputting step of outputting the selected frame as a reference image.

16. The image processing apparatus according to claim 1 , wherein the evaluation unit calculates the evaluation values by evaluating states of injection of the radiopaque dye for the plurality of frames of the digital subtraction angiography moving image.

17. The image processing apparatus according to claim 1 , wherein the evaluation unit evaluates a blood vessel region for each of the plurality of frames of the digital subtraction angiography moving image.

18. The image processing apparatus according to claim 17 , wherein the selection unit selects, as the reference image, a frame including a blood vessel region of a largest area among the plurality of frames.